The cost of aircraft operation depends in part on the fuel efficiency of the aircraft. A heavy aircraft requires more thrust and correspondingly consumes more fuel than a light aircraft. During the slowing phase of the landing of a flight, an aircraft will sometimes reverse the thrust direction of its engines to reduce its speed. Additionally, a braking system can be used to reduce speed of rotation of the landing gear assemblies. By reducing weight while slowing, less fuel can be consumed while reversing thrust, and wear and heat produced by braking can also be lessened. Thus, by reducing the weight of aircraft during any phase of flight, the cost to operate the aircraft can be reduced.
Typically, aircraft carry water aboard in a variety of locations and for different uses. As one example, aircraft carry potable water in storage tanks for consumption by passengers during the flight, either directly or through combination with other items as part of a meal. As another example, water is usually at least part of a fluid used to operate on-board toilets in the lavatory of the aircraft. Additionally, potable or non-potable water can be provided to wash basins in the lavatories. After use in the lavatory, the waste from the toilet or wash basin is usually stored in a separate waste tank until the aircraft has landed. The waste tank is then emptied prior to the next flight. Similarly, the water storage tanks are refilled after each flight. As a result, the aircraft carries hundreds of gallons and thousands of pounds in water and waste during the liftoff, cruise, and landing phases of each flight. Accordingly, part of the fuel used on each flight is to transport the water and waste tanks.
Water can also collect in fuel tanks of an aircraft, as condensation on the walls of less than fully-filled fuel tanks. Because water has a higher density than typical jet fuel, such as Jet A or Jet A-1, the water tends to sink in the fuel tank to a sump located near the bottom of the tank. During normal operation, scavenge pumps operate to remove the water from the fuel tanks, and store it as waste until the aircraft has landed and the waste tanks are emptied. The waste water from the fuel tanks creates additional weight, further increasing the cost of fuel to operate the aircraft on each flight.